tests: feature_backwards_compatibility.py test downgrade after upgrade
After upgrading the node, try to go back to the original version to make
sure that using a newver node version does not prevent the wallet file
from being downgraded again.

<pre>Merge #16902: O(1) OP_IF/NOTIF/ELSE/ENDIF script implementation
e6e622e5a0e22c2ac1b50b96af818e412d67ac54 Implement O(1) OP_IF/NOTIF/ELSE/ENDIF logic (Pieter Wuille)
d0e8f4d5d8ddaccb37f98b7989fb944081e41ab8 [refactor] interpreter: define interface for vfExec (Anthony Towns)
89fb241c54fc85befacfa3703d8e21bf3b8a76eb Benchmark script verification with 100 nested IFs (Pieter Wuille)
Pull request description:
While investigating what mechanisms are possible to maximize the per-opcode verification cost of scripts, I noticed that the logic for determining whether a particular opcode is to be executed is O(n) in the nesting depth. This issue was also pointed out by Sergio Demian Lerner in https://bitslog.wordpress.com/2017/04/17/new-quadratic-delays-in-bitcoin-scripts/, and this PR implements a variant of the O(1) algorithm suggested there.
This is not a problem currently, because even with a nesting depth of 100 (the maximum possible right now due to the 201 ops limit), the slowdown caused by this on my machine is around 70 ns per opcode (or 0.25 s per block) at worst, far lower than what is possible with other opcodes.
This PR mostly serves as a proof of concept that it's possible to avoid it, which may be relevant in discussions around increasing the opcode limits in future script versions. Without it, the execution time of scripts can grow quadratically with the nesting depth, which very quickly becomes unreasonable.
This improves upon #14245 by completely removing the `vfExec` vector.
ACKs for top commit:
jnewbery:
Code review ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54
MarcoFalke:
ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54 &#128052;
fjahr:
ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54
ajtowns:
ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54
laanwj:
concept and code review ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54
jonatack:
ACK e6e622e5a0e22c2ac1b50b96af818e412d67ac54 code review, build, benches, fuzzing
Tree-SHA512: 1dcfac3411ff04773de461959298a177f951cb5f706caa2734073bcec62224d7cd103767cfeef85cd129813e70c14c74fa8f1e38e4da70ec38a0f615aab1f7f7</pre>

Merge #18008: test: only declare a main() when fuzzing with AFL
b35567fe0ba3e6f8d8f9525088eb8ee62065ad01 test: only declare a main() when fuzzing with AFL (fanquake)
Pull request description:
This fixes fuzzing using [libFuzzer](https://llv...

Piecewise construct to avoid invalid construction
In CMainSignals::RegisterWithMempoolSignals running under Ubuntu 14.04
(QT 5.2), absent piecewise construction this fails to create the pair
because the argument is a connection, which is conver...